The present invention is directed to a metal-free, self-supporting optical cable which is utilized as an overhead cable in a field region between phase cables of a high-voltage aerial line. The cable is constructed as a weakly or slightly electrically conductive cable and has a cable core surrounded by at least one ply of high-strength filament material to form a carrier member and supporting element, and an outer cable cladding.
It is known to arrange optical overhead cables in the region of high-voltage aerial lines. A cable of this type is disclosed in U.S. Pat. No. 4,673,247, whose disclosure is incorporated by reference thereto and which claims priority from German Patent Application Nos. 34 24 047 and 35 04 041. The cable of this patent has a plurality of light waveguides which are accommodated inside of a tubular protective sheath and they are embedded in a filling compound composed of a soft, resilient, pasty material. At least one ply of high-tensile strength filament material is applied to this protective sheath and is preferably formed of an aramid thread. The cable formed in this way is provided with an external cable cladding.
U.S. Pat. No. 4,776,665, which corresponds to European Pat. No. A2 02 14 480, and whose disclosure is incorporated by reference, teaches that an overhead cable arranged in the field region between the phase cables of a high-voltage aerial field is fashioned with a weakly or slightly electrical conductivity in the cable core, namely so that the specific resistance of the cable core lies between 10.sup.5 ohm centimeter and 10.sup.10 ohm centimeter. In this way, undesirable damage to the cladding due to external or internal partial discharges or tracking currents are largely avoided.
Particularly when aramid threads are involved, the high tensile strength filament material is expediently pre-stretched and is radially compressed as far as possible, for example, is applied in a tight ply. These measures are important in order to largely anticipate the initial or "setting" dilatation of the filament material, to obtain electrically quasi-homogeneous, potentially weakly or slightly conductive member and in order to enable the reliable bracing of the cable on a non-yielding foundation.
A polyester foil has heretofore been used for the compressing armoring of the filament material. This polyester foil has been wrapped around the filament material and has been arranged in an overlapping relationship in a longitudinal direction. This kind of armoring and compressing of the filament material has a number of disadvantages. First, the foil to be employed for these purposes are relatively expensive. In addition, the foils tend to form folds or can even tear. Another disadvantage that occurs during manufacturing is that the firm application of the foil is only possible at a moderate speed. The foil strips have only an extremely limited running length (supply lengths) and, therefore, force frequent stopping of the manufacturing process. The foil and the cladding can, therefore, not be applied in one work cycle but require two manufacturing procedures following one another. Another disadvantage, that is of considerable importance given overhead cables in the region of electrical fields, is that the foils have relatively good insulating properties and, thus, present a disturbing insulator between the filament material, which is normally far lower in impedance and the non-tracking outside cladding. For example, the aramid threads or fibers of the above-mentioned U.S. Pat. No. 4,776,665 have a specific resistance on the order of magnitude of 10.sup.7 ohm centimeter, whereby the value of resistance on the order of magnitude of 10.sup.10 ohms centimeter can be counted on for the outer cladding. Foils of polyester material or other materials, which are considered by contrast, have a specific resistance on the order of magnitude of 10.sup.14 -10.sup.18 ohm centimeter and, thus, form an insulating layer between the cladding that is conductive to a certain extent and the filament material of the cable core of aramid threads, which has a far higher conductivity. Such a structure can lead to partial discharge at the foil, given corresponding field strengths.